A 10-year climatology of warm-season cloud patterns over Europe and the Mediterranean from Meteosat IR observations

Thermal infrared (IR, 10.5–12.5 micron) images from the Meteosat Visible and InfraRed Imager (MVIRI) of cold cloud episodes (cloud top brightness temperature < 241 K) are used as a proxy of precipitating clouds to derive a warm-season (May–August) climatology of their coherency, duration, span, and propagation speed over Europe and the Mediterranean. The analysis focuses over the 30–54N, 15W–40E domain in May–August 1996–2005. Harmonic analysis using discrete Fourier transforms is applied together with a statistical analysis and an investigation of the diurnal cycle. The objective of the study is to make available a set of results on the propagation dynamics of the cloud systems with the aim of assisting numerical modellers in improving summer convection parameterization. The zonal propagation of cold cloud systems is accompanied by a weak meridional component confined to narrow latitude belts. The persistence of cold clouds over the area evidences the role of orography, the Pyrenees, the Alps, the Balkans and Anatolia. A diurnal oscillation is found with a maximum marking the initiation of convection in the lee of the mountains and shifting from about 1400 UTC at 40E to 1800 UTC at 0 deg. A moderate eastward propagation of the frequency maximum from all mountain chains across the domain exists and the diurnal maxima are completely suppressed west of 5W. The mean power spectrum of the cold cloud frequency distribution evidences a period of one day all over Europe disappearing over the ocean (west of 10W). Other maxima are found in correspondence of 3 to 7 day synoptic activity. A median zonal phase speed of 16.1 m/s is found for all events >=1000 km and >=20 h and a full set of results divided by year and recurrence categories is also presented. The maxima of the diurnal signal are in phase with the presence of elevated terrain and with land masses.